1. Field of the Invention
The present invention relates to an optical recording carrier wherein a recording laser beam or the like is focused onto a recording material so that information signals can be recorded thereon, and to a method of producing such an optical recording carrier.
2. Description of Related Art
Nowadays, various studies are being conducted on the concept of an optical recording carrier wherein a laser beam is focused onto a recording material by a lens or the like, with the beam being varied in correspondence with information, so that information signals can be recorded on the recording material. In particular, the use of optical magnetic materials, Te-based materials and organic materials as recording materials has already been published. Of these, the Te-based recording materials are the nearest to being developed to a practical level, and some of them have already been put to practical use.
Te-based recording materials are classified into two groups. One group comprises Te-C, and Te-Se materials etc. as disclosed in U.S. Pat. No. 3,636,526, wherein a recording material is partially sublimated by the application of a laser beam thereon so a to form holes in the material, thereby recording signals. The other group comprises TeOx and Te-Sb-Se materials etc. as disclosed in U.S. Pat. Nos. 3,971,874 and 4,278,734, wherein signal recording is effected by utilizing the phenomenon that an amorphous-phase film made of the TeOx or Te-Sb-Se materials etc. is changed to a crystal-phase film by the heat of a laser beam.
However, these prior-art methods involves the following disadvantages.
In the former methods, featuring the thermal formation of recording holes, an air-gap system must be adopted to discharge the sublimated Te metal. This inevitably complicates the disc structure and makes the system difficult to assemble.
On the other hand, the latter method, employing a change from an amorphous phase to a crystal phase, has an advantage in that, since nothing is sublimated, the clad structure shown in FIG. 3 is enabled, and thus the disc structure can be simplified.
However, since the phase-change recording method, as mentioned previously, utilizes a change from an amorphous phase to a crystal phase of one of the recording materials such as TeOx, Te-Sb-Se and etc. some of which have already been practically used, there has existed such problem that a remarkable period of time is necessary till the recording material essentially reaches its final stable recorded condition (or stable crystal-phase condition). This problem is attributable to the fact that, when the recording material is changed from the amorphous phase to the crystal phase by the thermal energy of the laser beam, the crystallization progresses at a finite speed from crystal nuclei acting as seeds.
As an example, assume that, as shown in FIG. 4(a), an irradiation of laser beam 10 having a forcused diameter of about 1 .mu.m is applied onto a Te-based amorphous recording film 3 through a lens 11 for a short time (approximately 1 .mu.s). In this example, as shown in FIG. 4(b), only a region 12 is crystallized immediately after recording, but, for about two minutes after the recording, the crystallized region expands to another region 13. When a laser beam is used to record signals onto and immediately read them out of an optical recording carrier which includes a Te-based amorphous film deposited onto a disc-like substrate, while the carrier is rotating about its axis at high speed, the level of the reproduced signals gradually increases and the C/N ratio increase correspondingly (as indicated by a solid line in FIG. 5). The phenomenon is hereinunder referred to as "sensitization". Of course, the sensitization also depends upon the power level of the recording laser beam. Specifically, the time required for the sensitization phenomenon can be reduced by increasing the power level, but then signals cannot be accurately recorded since the final diameter of each recording pit becomes excessively large. If, in a recording material such as TeOx (0&lt;x&lt;2), the Te component is enriched, that is, if the value of x is reduced, the sensitization time can be reduced, but the weather resistance of the film deteriorates.
When optical recording and reproduction are effected by using a recording carrier having a Te-based amorphous recording film, the phenomomen of sensitization occurs when signals are reproduced. This is a significant problem in applications wherein it is necessary to read out signals at high speed immediately after they are recorded.